Circuit board having an interstitial inner via hole structure

ABSTRACT

A circuit board including a desired number of electrically insulating layers and wiring layers laminated alternately, and an inner via hole for securing an electrical connection between the wiring layers by compressing and hardening a conductive paste including a conductive particle and a resin. In the electrically insulating layer, a porous sheet is provided a resin sheet at least one surface, and the porous sheet is not impregnated with a resin at least at a central portion. A through hole penetrating the electrically insulating layer in the direction of the thickness of the electrically insulating layer is filled with a conductive paste including a conductive particle and a resin, and pores that are present inside the porous sheet are filled with laminated resin. The average hole diameter of the pores inside the porous sheet may be smaller than the average particle size of the conductive particle. Thereby, it is possible to make the insulating layer of the circuit board securing the interlayer electrically connection by an inner via hole including a conductive paste to be homogenized and ultra-thin and improve the reliability of the connection of the inner via hole.

FIELD OF THE INVENTION

The present invention relates to a circuit board used for electronicequipment, more specifically, it relates to a circuit board having aninterstitial inner via hole structure, a method for manufacturing thesame, and a circuit board electrically insulating material usedtherefor.

BACKGROUND OF THE INVENTION

In recent years, as miniaturization, light weight and high performanceof electronic equipment have been required, there has been a strongdemand not only for a small size and light weight of a circuit board butalso for higher speed for processing signals and high density mounting,and the like. In order to meet such demands, in the field of a circuitboard technology, a technology for increasing the number of layers in alaminate structure, a technology of making the diameter of an inner viahole smaller, a technology for making circuit patterns finer, and thelike have been developed rapidly, and circuit boards having variousstructures have been proposed or put into practice. As one example, thepresent assignee has developed a circuit board having an all-layer IVH(interstitial inner via hole) structure, in which the interlayerelectric connection is secured with an inner via hole filled with aconductive paste in place of a through hole structure that has been ausual method in conventional circuit boards (see, for example, JP7(1995)-14746 4 A). This circuit board has features that no through holeis needed and inner via holes can be arranged at arbitrary positions,and therefore, this circuit board has a high capacity of wiring, a highdegree of freedom in design, shortening of wiring, etc. Thus, thiscircuit board is suitable for signal processing at high speed and highdensity mounting. In this circuit board, the conductive paste filled inthe through hole is compressed and hardened simultaneously with moldingof the laminate, thereby providing an inner via hole for securing aninterlayer electrical connection. Therefore, this circuit board needs anelectrically insulating material having such a compressive performancethat permits sufficient compression of a conductive paste and is capableof maintaining the shape of the inner via hole. In general, as aconductive paste, a prepreg obtained by impregnating an aramid non-wovenfabric with a thermosetting resin is used. The compressive performanceof the aramid prepreg can be achieved by the melt flow of thethermosetting resin and the effect of defoaming of any remaining airfoam. The maintenance of the shape of the inner via hole can be achievedby rigid property of the aramid non-woven fabric.

In the future, in a view that a circuit board will increasingly berequired to have a finer circuit pattern, a smaller diameter of an innervia hole and an increase in number of the layers of the laminatestructure and the like, it is extremely important to make theelectrically insulating layer to be homogenized and ultra-thin. However,a non-woven fabric made of an aramid staple fiber having a diameter of10-20 μm may cause a non-negligible non-uniformity of the physicalproperty in a micro area depending on the direction of the fiber.Furthermore, a satisfactory thin layer cannot be obtained because thereis a limitation to the number of fibers in the thickness direction.

SUMMARY OF THE INVENTION

With the foregoing in mind, it is an object of the present invention toprovide a circuit board electrically insulating material and a circuitboard, in which an electrically insulating layer is made to behomogenized and ultra-thin and the connection reliability of the innervia hole including a conductive paste is improved so as to achievehigher density mounting, and also provide a method for manufacturing thesame.

In order to achieve the above-mentioned objectives, the circuit boardelectrically insulating material of the present invention is a circuitboard electrically insulating material in sheet form, insulating sheet,including a porous sheet; and a resin layer laminated to a surface ofthe porous sheet, wherein at least a central portion of the porous sheetis not impregnated with resin and the circuit board insulating materialhas a thickness sufficiently small for use in a circuit board. In otherwords, the circuit board electrically insulating material of the presentinvention includes a porous sheet provided with a semi-hardened resinlayer on both surfaces or one surface. In the circuit board electricallyinsulating material, the pores of the porous sheet are hollow.Furthermore, the pores of the porous sheet of the present invention maybe formed by continuous holes. Hereinafter, “at least a center of theporous sheet is not impregnated with resin” also is referred to as“pores of the porous sheet are hollow.”

Next, the circuit board of the present invention includes a desirednumber of electrically insulating layers and wiring layers laminatedalternately, and an inner via hole for securing an electrical connectionbetween the wiring layers by compressing and hardening a conductivepaste including a conductive particle and a resin; wherein theelectrically insulating layer includes a porous sheet in which a resinlayer is laminated to at least one surface, and at least a centralportion of the porous sheet is not impregnated with a resin; a throughhole penetrating the electrically insulating layer in the thicknessdirection of the electrically insulating layer is filled with aconductive paste including a conductive particle and a resin, and poresthat are present inside the porous sheet are filled with the laminatedresin; and the average hole diameter of the pores that are presentinside the porous sheet is smaller than the average particle size of theconductive particle.

Next, a first method for manufacturing a double-sided circuit board ofthe present invention includes: providing a through hole at a desiredposition of a laminate in which mold release films are formed on bothsurfaces of the above-mentioned circuit board electrically insulatingmaterial; filling the through hole with a conductive paste; peeling offthe mold release film from the laminate in which the through hole isfilled with the conductive paste; superimposing metal foils on bothsurfaces of the circuit board electrically insulating material fromwhich the mold release films have been peeled off to form a laminate;heating and pressing the laminate to allow hollow pores of the poroussheet to be filled with a resin and allow the metal foils to be adheredto the porous sheet while compressing and hardening the conductive pastefilled in the through hole, thereby providing an inner via hole; andforming desired circuit patterns on the metal foil.

Next, a second method for manufacturing a double-sided circuit board ofthe present invention includes forming a laminate either bysuperimposing resin sheets on both surfaces of a porous sheet that isnot completely impregnated with a resin, further superimposing the moldrelease films on both surfaces of the laminated resin sheet, andpressing thereof; or by forming a resin layer on one surface of a moldrelease film, further sandwiching a porous sheet that is not impregnatedwith a resin by the sides of the resin layer of the mold release filmsprovided with the resin layers; and pressing thereof; providing athrough hole in a desired position of the laminate provided with themold release films; filling the through hole with a conductive paste;peeling off the mold release films from the laminate in which thethrough hole has been filled with the conductive paste; superimposingthe metal foils on both surfaces of the laminate from which the moldrelease films have been peeled off; heating and pressing the laminate toallow hollow pores of the porous sheet to be filled with a resin andallow the metal foil to be adhered to the porous sheet, and compressingand hardening the conductive paste filled in the through hole, therebyproviding an inner via hole; and forming desired circuit patterns on themetal foil.

Next, a third method for manufacturing a double-sided circuit board ofthe present invention includes providing a through hole at a desiredposition of a laminate in which mold release films are formed on bothsurfaces of a porous sheet that is not completely impregnated with aresin; filling the through hole with a conductive paste, and peeling offthe mold release films from the laminate in which the through hole hasbeen filled with the conductive paste; sandwiching the laminate fromwhich the mold release films have been peeled off by transfer media, thetransfer media being formed on a supporting base material by filling aresin in an exposed portion of the supporting base material at the sideof the wiring layer provided on the supporting base material; andheating and pressing the laminate to allow hollow pores of the poroussheet to be filled with a resin and allow the wiring layer to be adheredto the porous sheet, and compressing and hardening the conductive pastefilled in the through hole, thereby providing an inner via hole; andremoving the supporting base material of the transfer medium from theheated and pressed laminate.

Next, a fourth method for manufacturing a double-sided circuit board ofthe present invention includes forming a resin layer at the side of awiring layer of a transfer medium in which a wiring layer is provided ona supporting base material; sandwiching both surfaces of a porous sheetthat is not completely impregnated with a resin between the transfermedium on which the resin layer is formed and a mold release filmprovided with a resin layer to form a laminate; forming a non-throughhole at a desired position of the laminate from the side of the moldrelease film; filling the non-thorough hole with a conductive paste;peeling off the mold release film from the laminate in which thenon-through hole has been filled with the conductive paste;superimposing a metal foil on the surface of the laminate from which themold release film has been peeled off; heating and pressing the laminateto which the metal foil is superimposed to allow hollow pores of theporous sheet to be filled with a resin and allow the metal foils and thewiring layer of the transfer medium to be attached to the porous sheet,and compressing and hardening the conductive paste filled in thenon-through hole, thereby providing an inner via hole; forming desiredcircuit patterns on the metal foil; and removing the supporting basematerial from the laminate in which the circuit patterns have beenformed on the metal foil.

Next, a first method for manufacturing a multilayer circuit board of thepresent invention repeats the following steps once or more: providing athrough hole at a desired position of the laminate in which mold releasefilms are formed on both surfaces of the above-mentioned circuit boardelectrically insulating materials; and filling the through hole with aconductive paste; peeling off the mold release films from the laminatein which the through hole has been filled with the conductive paste;superimposing a desired number of the circuit board electricallyinsulating material from which the mold release films have been peeledoff and the circuit board provided with two or more of wiring layersalternately so that the circuit board electrically insulating materialbecomes the outermost layer, and furthermore superimposing metal foilson the surface thereof to form a laminate; heating and pressing thelaminate to allow hollow pores of the porous sheet to be filled with aresin and allow the metal foil and the circuit board to be adhered tothe porous sheet, and compressing and hardening the conductive pastefilled in the through hole, thereby providing an inner via hole; andforming desired circuit patterns on the metal foil.

Next, a second method for manufacturing a multilayer circuit board ofthe present invention repeats the following steps once or more: forminga laminate either by superimposing resin sheets on both surfaces of aporous sheet that is not completely impregnated with a resin, furthersuperimposing the mold release films on both surfaces of the laminatedresin sheet, and pressing thereof; or by forming a resin layer on onesurface of the mold release film, sandwiching a porous sheet that is notcompletely impregnated with the resin between the sides of the resinlayer of the mold release film provided with the resin layer, andpressing thereof; providing a through hole at a desired position of thelaminate provided with the mold release films, filling the through holewith the conductive paste, and peeling off the mold release films fromthe laminate in which the through hole has been filled with theconductive paste; superimposing a desired number of the laminate fromwhich the mold release films have been peeled off and the circuit boardelectrically insulating material provided with two or more of the wiringlayers alternately so that the circuit board electrically insulatingmaterial becomes the outermost layer, and furthermore superimposing ametal foil; heating and pressing the laminate to allow hollow pores ofthe porous sheet to be filled with a resin and allow the metal foil andthe circuit board to be adhered to the porous sheet, and compressing andhardening the conductive paste filled in the through hole, therebyproviding an inner via hole; and forming desired circuit patterns on themetal foil.

Next, a third method for manufacturing a multilayer circuit board of thepresent invention repeats the following steps once or more: forming aresin layer on at least one surface of the circuit board provided withtwo or more wiring layers; superimposing a porous sheet that is notcompletely impregnated with a resin at the side of the resin layer ofthe circuit board; and furthermore superimposing the mold release filmprovided with the resin layer on one surface onto the porous sheet;forming a non-through hole at a desired position of the laminate fromthe side of the mold release film; filling the non-through hole with theconductive paste; and peeling off the mold release film from thelaminate in which the non-through hole has been filled with theconductive paste; superimposing metal foils on the surfaces of thelaminate from which the mold release films have been peeled off; heatingand pressing the laminate to allow hollow pores of the porous sheet tobe filled with a resin and allow the metal foil and the circuit board tobe adhered to the porous sheet, and compressing and hardening theconductive paste filled in the through hole, thereby providing an innervia hole; and forming desired circuit patterns on the metal foil.

As mentioned above, with the circuit board electrically insulatingmaterial, circuit boards or the methods for manufacturing a circuitboard of the present invention, it is possible to make the electricallyinsulating layer of the circuit board which secures the electricallyconnection with the inner via hole including a conductive paste to behomogenized and ultra thin and the reliability of the via connection tobe higher. Therefore, the present invention can provide a circuit boardthat is extremely suitable for high density mounting.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically showing a circuit boardelectrically insulating material according to a first embodiment of thepresent invention.

FIG. 2 is a cross-sectional view schematically showing a circuit boardaccording to a second embodiment of the present invention.

FIGS. 3A-3B are cross-sectional views schematically showing a mechanismof the via connection of the present invention; FIG. 3A is across-sectional view schematically showing an inner via hole beforepress molding; and FIG. 3B is a cross-sectional view schematicallyshowing an inner via hole after press molding.

FIGS. 4A to 4F are cross-sectional views schematically showing the stepsof a method for manufacturing a double-sided circuit board according toa third embodiment of the present invention.

FIGS. 5A to 5B are cross-sectional views schematically showing the stepsof a method for manufacturing a laminate according to a fourthembodiment of the present invention.

FIGS. 6A to 6F are cross-sectional views schematically showing a methodfor manufacturing a double-sided circuit board according to a fifthembodiment of the present invention.

FIGS. 7A to 7I are cross-sectional views schematically showing a methodfor manufacturing a double-sided circuit board according to a sixthembodiment of the present invention.

FIGS. 8A to 8C are cross-sectional views schematically showing a methodfor manufacturing a double-sided circuit board according to a seventhembodiment of the present invention.

FIGS. 9A to 9H are cross-sectional views schematically showing a methodfor manufacturing a double-sided circuit board according to a ninthembodiment of the present invention.

FIGS. 10A and 10B are cross-sectional views schematically showing amechanism of the via connection of the present invention; FIG. 10A is across-sectional view schematically showing an inner via hole beforepress molding; and FIG. 10B is a cross-sectional view schematicallyshowing an inner via hole after press molding.

DETAILED DESCRIPTION OF THE INVENTION

In a porous sheet used for a circuit board electrically insulatingmaterial of the present invention, all the pores are not necessarilyhollow. It is preferable that the ratio of pores (the porosity) is 10vol. % or more with respect to the entire volume of the circuit boardelectrically insulating material. More preferably, the porosity is inthe range from 10 vol. % to 45 vol. %.

The average hole diameter of the pores is preferably in the range from0.1 μm to 10 μm. The pores can be measured by mercury porosimetry with amercury porosimeter “Autopore 3” produced by Micrometrictics, a mercurypressure process, and the observation of pores in the cross section by ascanning electron microscope.

Furthermore, on at least one surface of the porous sheet, a resin layeris laminated, a part of the porous sheet may be impregnated with theresin. The preferable ratio of the part that is not impregnated with theresin is 50% to 90% in the thickness direction.

It is preferable that the thickness of the porous sheet is in the rangefrom 5 μm to 100 μm.

It is preferable that the porous sheet is at least one selected from anorganic material and ceramics.

Furthermore, it is preferable that the organic material is at least onematerial selected from polytetrafluoroethylene (PTFE), polyimide, allaromatic polyamide, and all aromatic polyester. Herein, for example, amaterial such as PET that has an alkyl group in the chain is notincluded in the all aromatic polyester.

Furthermore, it is preferable that the porous sheet is a non-wovenfabric containing a synthetic fiber as a main component.

Furthermore, in the first to third methods for manufacturing amultilayer circuit board, it is preferable that the conductive pastecontains a conductive particle and a resin as a main component and theaverage hole diameter of the pores of the porous sheet is smaller thanthe average particle size of the conductive particle.

In the circuit board electrically insulating material of the presentinvention, all of the pores of the porous sheet are not necessarilyhollow. The lower limit of the ratio of pores (the porosity) is 10 vol.% with respect to the entire volume of the circuit board electricallyinsulating material. However, the porosity is not limited to this aloneas long as the porosity can secure a suitable compressive rate forsecuring the satisfactory connection reliability of the inner via holeswhen the circuit board electrically insulating material is used for acircuit board in which the interlayer connection of the inner via holewith the use of a conductive paste. Furthermore, an upper limit of thehole diameter of the pores may be 10 μm. However, the hole diameter isnot limited to this alone, especially if the hole diameter is smallerthan the particle size of the conductive particle when it is used forthe circuit board in which the inner via hole including a conductivepaste is used for interlayer connection. By the below mentioned filtereffect, it is possible to form an inner via hole having high connectionreliability. Furthermore, it is preferable that the porous sheet of thecircuit board electrically insulating material of the present inventionincludes an organic material or ceramics. The organic material issuitable in the case where the circuit board is required to have a lightweight, low dielectric constant and flexibility. Ceramics are suitablein the case where the circuit board is required to have a low thermalexpansion, high thermal resistance and high rigidity. Furthermore, it ispreferable that the organic porous sheet is selected from the groupconsisting of polytetrafluoroethylene (PTFE), polyimide, all aromaticpolyamide, and all aromatic polyester. These materials are excellent inthermal stability and an electrically insulating property and has a lowdielectric property, and thus, they are suitable for an electricallyinsulating layer of the circuit board. Furthermore, the porous sheet ofthe circuit board electrically insulating material may be a non-wovenfabric containing an organic fiber as a main component. In this case, itis possible to radically improve the connection reliability of the innervia holes as compared with the conventional circuit board (which uses anon-woven fabric for a reinforcing material) in which an inner via holeincluding a conductive paste. Furthermore, the resin layer of thecircuit board electrically insulating material of the present inventionis required to be a resin that is melt-flown and hardened when thecircuit board is molded by heating and pressing (mainly vacuum heatpress molding), that is, a semi-hardened resin. In particular, athermosetting resin such as an epoxy resin, a phenol resin, a polyimideresin etc. or a denatured thermosetting resin is preferred.

Furthermore, it is preferable that the conductive paste contains aconductive particle and a resin as a main component and that the averagehole diameter of the pores of the porous sheet is smaller than theaverage particle size of the conductive particle. Herein, “maincomponent” denotes a component occupying 60 weight % or more withrespect to the whole. The same is true in the other cases of the presentinvention.

Furthermore, it is preferable that the conductive paste includes theconductive particle in the range from 70 to 95 weight % and the resin isin the range from 5 to 30 weight %.

Furthermore, the circuit board electrically insulating material of thepresent invention is explained in more detail by way of Embodiments withreference to drawings.

[First Embodiment]

FIG. 1 is a cross-sectional view schematically showing a circuit boardelectrically insulating material according to a first embodiment of thepresent invention. This circuit board electrically insulating material101 includes a porous sheet 102 and semi-hardened resin layers 103. Thesemi-hardened resin layers are formed on both surfaces of the poroussheet 102. Pores of the porous sheet 102 are not completely filled witha resin. See also FIGS. 10A and 10B. FIG. 10 B shows the pores notimpregnated 310 after press molding. This creates a central portion 311of the porous sheet that is not impregnated with resin.

In the configuration of the first embodiment, when the circuit boardelectrically insulating material 101 of the present invention is heatedand pressed, the melt flow of the resin layer 103 occurs due to the heatand the resin of the resin layer is allowed to fill in the pores of theporous sheet 102. In other words, since the resin layer 103 enters thepores of the porous sheet 102, the electrically insulating material 101can be changed significantly in the thickness, i.e., the electricallyinsulating material 101 can be compressed extremely excellently due toheat and pressure. Therefore, when the circuit board electricallyinsulating material 101 is used for a circuit board in which interlayerconnection is performed with an inner via hole including a conductivepaste, the conductive paste is compressed quite sufficiently, thusenhancing the reliability of the via connection.

Conventionally, in order to secure the compressive performance,conventionally, a non-woven fabric that is a reinforcing material alsois required to have a compressive performance to some extent. In theconfiguration of the present invention, since the compressiveperformance can be secured by the fact that the resin layer enters theinside of the porous sheet, the reinforcing material in not necessarilycompressed. Therefore, since a non-woven fabric that hardly has acompressive performance, in order words, a non-woven fabric havinghigher density (i.e., a higher bulk density) can be used, anelectrically insulating material suitable for a circuit board for highdensity mounting can be provided. Furthermore, in the configuration ofthe present invention, an inorganic porous sheet having no compressiveperformance can be used as a reinforcing material for the circuit boardelectrically insulating material having a compressive performance.

Furthermore, by using a material having micro pores, preferably poreshaving a hole diameter of 10 μm or less, for the porous sheet 102, it ispossible to make the electrically insulating layer to be homogenized andultra-thin, and to make a connection by an inner via hole including aconductive paste more reliable easily. Thus, it is possible to provide acircuit board suitable for high density mounting.

The following is an explanation of a circuit board of the presentinvention. The circuit board of the present invention has a layer forsecuring the electrical connection between the wiring layers byproviding an inner via hole by compressing and hardening a conductivepaste including a conductive particle and a resin. The electricallyinsulating layer provided with the inner via hole includes a compositematerial made of a porous sheet and a resin.

Furthermore, the hole diameter of the pores of the porous sheet may besmaller than the particle size of the conductive particles. Furthermore,the electrically insulating layer with the inner via hole may beprovided in one process, i.e., by carrying out a process of filling theresin into the pore of the porous sheet and a process of compressing andhardening the conductive paste at the same time. Furthermore, it ispreferable that the porous sheet of the circuit board of the presentinvention has a hole diameter of 10 μm as an upper limit and is made ofan organic sheet or ceramic sheet. The organic sheet is suitable forachieving light weight, low dielectric constant, and flexibility of thecircuit board. The ceramic sheet is suitable for achieving low thermalexpansion, high heat resistance, and high rigidity of the circuit board.Furthermore, it is preferable that the organic porous sheet is selectedfrom the group consisting of polytetrafluoroethylene (PTFE), polyimide,all aromatic polyamide, and all aromatic polyester. Since they areexcellent in the thermal stability and insulation performance, and lowdielectric constant, they are suitable for an electrically insulatinglayer of the circuit board. Furthermore, it is preferable that theconductive paste usable for the circuit board of the present inventionincludes a conductive particle selected from gold, silver, copper andpalladium, or an alloy thereof, and a thermosetting resin, specifically,an epoxy resin. Hereinafter, the circuit board of the present inventionis described more specifically by way of embodiments with reference todrawings.

[Second Embodiment]

FIG. 2 is a cross-sectional view schematically showing a configurationof a circuit board according to a second embodiment of the presentinvention. A circuit board 201 is a four-layered circuit board includinga double-sided circuit board 202 and electrically insulating layers 203,formed by filling a resin in pores of the porous sheet, which are formedon both surfaces of the double-sided circuit board 202. In thefour-layered circuit board, the interlayer electrical connection betweeninner wiring layer 205 is secured by the through hole 204 and theinterlayer electrical connection between the inner wiring layer 205, andan outer wiring layer 206 is secured by an inner via hole 207 filledwith a conductive paste. According to this configuration, it is possibleto provide a circuit board extremely suitable for high density mountingbecause an IVH structure homogeneous and thin electrically insulatinglayer is provided at the outermost layer, and the inner via holes of theelectrically insulating layer can be made small easily.

In the second embodiment, the inner via hole 207 is described in moredetail with reference to FIGS. 3A and 3B. FIG. 3A is a cross-sectionalview schematically showing an inner via hole before the circuit board ispress molded. The semi-hardened resin layers 302 are provided on bothsurfaces of the porous sheet 301, and furthermore, the wiring layers 303are provided on the surfaces of both resin layers 302, thereby forming alaminate. A through hole 304 provided on the laminate is filled with aconductive paste including a conductive particle 305 and a thermosettingresin 306. By heating and pressing this laminate, the structure ischanged from the structure of FIG. 3A into the structure of FIG. 3B.FIG. 3B corresponds to the inner via hole 207 of the second embodiment.In this structure, the electrically insulating layer 307 formed byfilling the pores of the porous sheet with the resin (resin of the resinlayer 302) has an inner via hole 308 provided by compressing andhardening the conductive paste including the conductive particle 305 andthe thermosetting resin 306. In the embodiment of FIG. 3A, the holediameter of the pores of the porous sheet 301 may be smaller than theaverage particle size of the conductive particle 305. Therefore, whenthe conductive paste is subjected to heat and pressure, the conductiveparticle 305 in the conductive paste remains in the through hole 304perfectly, and an excessive paste resin is ejected to the electricallyinsulating layer 301. Thus, a filter effect can be obtained. As aresult, the F value (weight ratio of the conductive particles in thepaste) of the inner via hole 308 after being molded can be increasedradically, and the inner via hole 308 can have a highly reliableconnection. In the heating and pressing molding shown in FIGS. 3A to 3B,the inner via hole 308 is compressed sufficiently because the conductivepaste undergoes large compression effect and because the resin of theresin layer 302 is filled in the pores of the porous sheet 301.Furthermore, in FIG. 3A, when the through hole 304 is filled with theconductive paste, the resin in the paste can be ejected to the pores inadvance by a capillary phenomenon. Therefore, even in a stage beforemolding, the F-value of the paste filled in the through hole 304 can beincreased. The above-mentioned function can be provided in the casewhere the pores of the porous sheet are hollow as shown in FIG. 3A, andthus the inner via hole 308 can have an extremely high connectionreliability.

Next, a method for manufacturing a double-sided circuit board of thepresent invention is described. The method for manufacturing thedouble-sided circuit board of the present invention employs aninter-layer connecting technique using an inner via hole including aconductive paste. The method is characterized by carrying outcompressing and hardening a conductive paste; filling a resin in thepores of the porous sheet; and molding a circuit board in one step. Thismethod makes it possible to compress the conductive paste sufficiently.Furthermore, since resin in the paste can be ejected to the pores of theporous sheet by a capillary phenomenon when the through holes ornon-through holes are filled with the conductive paste, the F value ofthe conductive paste can be increased in advance. Therefore, it ispreferable that the hole diameter of the pores of the porous sheet issmaller than the particle size of the conductive particle. It is morepreferable that the upper limit of the hole diameter is 10 μm. Moreover,this effect can be enhanced by filling the conductive paste in a vacuum.As mentioned above, by the method for manufacturing the double-sidedcircuit board of the present invention, it is possible to provide adouble-sided circuit board capable of making the electrically insulatinglayer to be homogenized and ultra-thin and making the connection of theinner via hole including a conductive paste to be more reliable.Moreover, the porous sheet may be a non-woven fabric containing anorganic fiber as a main component. In this case, it is possible toprovide a circuit board having more reliable connection of the inner viahole and dimensional stability as compared with those of a conventionalcircuit board using an aramid prepreg. Furthermore, it is preferable inthe double-sided circuit board of the present invention that theconductive paste contains a conductive paste selected from gold, silver,copper and palladium or an alloy thereof and a thermosetting resin, suchas an epoxy resin etc. Hereinafter, the method for manufacturingdouble-sided circuit boards of the first to fourth embodiments aredescribed with reference to drawings.

[Third Embodiment]

A method for manufacturing a first double-sided circuit board of thepresent invention is described. FIGS. 4A to 4F are cross-sectional viewsschematically showing a method for manufacturing a double-sided circuitboard according to a third embodiment of the present invention. Themethod for manufacturing the first double-sided circuit board includessteps of: providing a through hole 405 on a laminate 404 in which moldrelease films 403 are formed on both surfaces of the circuit boardelectrically insulating material of the present invention in whichsemi-hardened resin layers 402 are formed on both surfaces of a poroussheet 401 (see FIG. 4A); filling the through hole with a conductivepaste 406 (see FIG. 4B); peeling off only the mold release films 403from the laminate 407 in which the through hole has been filled with theconductive paste (see FIG. 4C); superimposing metal foils 409 on bothsurfaces of the circuit board electrically insulating material 408 fromwhich the mold release films have been peeled off (see FIG. 4D); heatingand pressing the laminate of FIG. 4D to allow hollow pores of the poroussheet to be filled with a resin and allow the metal foils to be adheredto the porous sheet, and further compressing and hardening theconductive paste to form an inner via hole 410 (see FIG. 4E); andforming desired circuit patterns 413 on the metal foil (see FIG. 4F).

[Fourth Embodiment]

A method for manufacturing a second double-sided circuit board of thepresent invention is described. FIGS. 5A to 5B are cross-sectional viewsschematically showing a method for manufacturing a double-sided circuitboard according to a third embodiment of the present invention. Themethod for manufacturing the double-sided circuit board does notnecessarily use a circuit board electrically insulating material of thepresent invention. The method for manufacturing the second double-sidedcircuit board includes either a step of superimposing semi-hardenedresin sheets 502 on both surfaces of the porous sheet 501 having hollowpores, and further superimposing the mold release films 503 on thesurface of the both resin sheets 502, and pressing thereof to form alaminate 504 (see FIG. 5A); or the step of forming a semi-hardened resinlayer 505 on one surface of the mold release film 503, and thensandwiching the porous sheet 501 having hollow pores between the resinlayer sides of the mold release films, and then pressing thereof to forma laminate 504 (see FIG. 5B). By employing any one of theabove-mentioned steps, the laminate that is the same as the laminate 404in FIG. 4A can be obtained. The subsequent steps including providing athrough hole to the formation of the circuit patterns are the same as inthe third Embodiment.

[Fifth Embodiment]

A method for manufacturing a third double-sided circuit board of thepresent invention is described. FIGS. 6A to 6F are cross-sectional viewsschematically showing a method for manufacturing a double-sided circuitboard according to a fifth embodiment of the present invention. Themethod for manufacturing the third double-sided circuit board includessteps of: providing a through hole 604 on a predetermined position of alaminate 603 in which mold release films 602 are formed on both surfacesof a porous sheet 601 having hollow pores (see FIG. 6A); filling thethrough hole with a conductive paste 605 (see FIG. 6B); peeling off themold release films 602 from the laminate 606 in which the through holehas been filled with the conductive paste (FIG. 6C); sandwiching thelaminated 607 from which the mold release films have been peeled off bytransfer media 608 formed on support base materials 609 (FIG. 6D);heating and pressing the laminate of FIG. 6D to allow hollow pores ofthe porous sheet to be filled with the resin provided for the transfermedia and allow the wiring layer to be adhered to the porous sheet, andfurther compressing and hardening the conductive paste that has beenfilled in the through hole to form an inner via hole 614 (see FIG. 6E);and removing the support base materials 609 of the transfer medium fromthe molded laminate 613 (see FIG. 6F). According to this embodiment, asshown in FIGS. 6D to 6F, since the wiring layers having a predeterminedcircuit patterns are transferred, it is possible to provide adouble-sided circuit board excellent in coplanarity (flatness) andextremely suitable for surface mounting. Furthermore, in thisembodiment, the conductive paste is compressed with the convex wiringlayer, so that a more compressed inner via hole can be obtained, and itis possible to provide a circuit board having a highly reliableconnection. In the step shown in FIG. 6A, a one-side adhesive film maybe used for the mold release film 602. For forming the semi-hardenedresin layer 611 on the transfer medium 608, a printing method such as ascreen printing, etc. may be used.

[Sixth Embodiment]

A method for manufacturing a fourth double-sided circuit board of thepresent invention is described. FIGS. 7A to 7I are cross-sectional viewsschematically showing a method for manufacturing a double-sided circuitboard according to a sixth embodiment of the present invention. Themethod for manufacturing the fourth double-sided circuit board includesat least forming a semi-hardened resin layer 703 on the wiring layerside of a transfer medium laminate in which a wiring layer 702 is formedon a support base material 701 (see FIG. 7A); sandwiching a porous sheet707 having hollow pores between a mold release film 706 provided with atransfer medium 704 and a semi-hardened resin 705 (see FIG. 7B);providing a non-through hole 709 from the side of the mold release filmof the laminate 708 (see FIG. 7C); filling the non-through hole with aconductive paste 710 (see FIG. 7D); peeling off only the mold releasefilms 706 from the laminate 711 filled with the conductive paste (seeFIG. 7E); superimposing a metal foil 713 on a laminate 712 from whichthe mold release films is peeled off (see FIG. 7F); heating and pressingthe laminate of the FIG. 7F to allow hollow pores of the porous sheet tobe filled with a resin and allow the metal foil to be adhered to theporous sheet and further compressing and hardening the conductive pasteto form an inner via hole 714 (see FIG. 7G); forming a predeterminedcircuit pattern 718 on the metal foil of the molded laminate (see FIG.7H); and removing only the support base material 701 from the laminateof FIG. 7H (see FIG. 7I). According to this embodiment, as shown in FIG.7C, since the non-through hole 709 is provided while reading out thecircuit patterns, it is not necessary to carry out the positioning ofthe circuit patterns and the inner via hole when the laminate is formed.Thus, the method for manufacturing the double-sided circuit board ofthis embodiment is extremely excellent in allowing the circuit patternto be finer. Furthermore, although the manufacturing process is carriedout on one side in this embodiment, similar to the fifth embodiment, aconductive paste is compressed with the convex wiring layer 702.Therefore, it is possible to obtain a more compressed inner via hole,and to provide a circuit board having a highly reliable connection.Moreover, in the step of FIG. 7A, the resin layer 703 may be printed bya spin coating method, a printing method, or the like.

Next, a method for manufacturing multilayer circuit board according tothe present invention is described. The method for manufacturing themultilayer circuit board of the present invention is a method formanufacturing the multilayer circuit board using an interlayerconnecting technique with the inner via hole including a conductivepaste. In the formation of the layer having the inner via hole includinga conductive paste, a compressing and hardening of the conductive paste,filling pores of the porous sheet with a resin, and forming themulti-layer structure are carried out in one step. Thereby, theconductive paste can be compressed sufficiently. Furthermore, in thestep of filling a through hole or a non-through hole with the conductivepaste, a resin in the paste is ejected to the pores of the porous sheetby a capillary phenomenon, and the F value of the conductive paste canbe increased in advance. Therefore, it is preferable that the holediameter of the pores of the porous sheet is smaller than the particlesize of the conductive particle. Preferably, the upper limit of the holediameter is set to be 10 μm. The above-mentioned effect is enhanced whenfilling of the conductive paste is carried out in a vacuum. As mentionedabove, according to the method for manufacturing a multilayer circuitboard of the present invention, it is possible to provide a multilayercircuit board capable of making the electrically insulating layer to behomogenized and ultra-thin and easily making the connection by an innervia hole including a conductive paste to be more reliable. Moreover, theporous sheet may be a non-woven fabric containing an organic fiber as amain component. In this case, it is possible to provide a circuit boardhaving more reliable connection of the inner via hole and dimensionalstability as compared with those of a conventional circuit board usingan aramid prepreg. Furthermore, it is preferable in the multilayercircuit board of the present invention that the conductive pastecontains a conductive material selected from gold, silver, copper andpalladium or, an alloy thereof, and thermosetting resin such as an epoxyresin as a main component. Hereinafter, the method for manufacturing themultilayer circuit boards of the first to fourth embodiments isdescribed with reference to the drawings.

[Seventh Embodiment]

A method for manufacturing a first method for manufacturing a multilayerdouble-sided circuit board of the present invention is described. FIGS.8A to 8C are cross-sectional views schematically showing a method formanufacturing a multilayer circuit board according to a seventhembodiment of the present invention. The method for manufacturing thefirst double-sided circuit carries out at least the steps of:superimposing a laminate 801 from which a mold release film has beenpeeled off and a circuit board 802 alternately, and furthersuperimposing a metal foil 803 thereon (see FIG. 8A); heating andpressing the laminate to allow hollow pores of the porous sheet to befilled with the resin and allow the metal foil 803 to be adhered to thecircuit board 802, and further compressing and hardening the conductivepaste 804 to form an inner via hole 807 (see FIG. 8B), and formingcircuit patterns 810 on the metal foil (see FIG. 8C). Theabove-mentioned steps are repeated once or more. By repeating a seriesof the above-mentioned steps, it is possible to provide a multi-layercircuit board in which many layers are laminated. The laminate 801corresponds to the laminate 407 of FIG. 4B, which can be manufactured bythe step explained in the method for manufacturing the firstdouble-sided circuit board of the present invention. According to thisembodiment, since a fine pattern layer can be formed on the outermostlayer easily, it is possible to provide a multilayer circuit board forhigh density mounting. Moreover, the circuit board 802 of thisembodiment uses a circuit board having a through hole, however, thecircuit board 802 of the present invention may have other configuration.

[Eighth Embodiment]

This embodiment is the same as in the seventh embodiment except that asthe laminate 801 of FIG. 8A, the laminate 504 described in the methodfor manufacturing the second double-sided circuit board (FIG. 5B) isused, and it possible to provide a multilayer circuit board for highdensity mounting.

[Ninth Embodiment]

The method for manufacturing a third multilayer circuit board of thepresent invention is described. FIGS. 9A to 9H are cross-sectional viewsschematically showing a method for manufacturing the multilayer circuitboard of the ninth embodiment. The method of this embodiment includesthe steps of: forming a semi-hardened resin layer 902 on one surface ofa circuit board 901 provided with a support base material 907, an innerwiring layer 906 and an inner via hole 904 including a conductive pasteto form a circuit board 903 (see FIG. 9A); superimposing a porous sheet908 having hollow pores at the side of the resin layer, and furthermoresuperimposing a mold release film 910 provided with a semi-hardenedresin layer 909 to form a laminate 911 (see FIG. 9B); providing anon-through hole 912 from the side of the mold release film of thelaminate 911 (see FIG. 9C); filling the non-through hole with aconductive paste 913 (see FIG. 9D); peeling off only the mold releasefilm 910 from the laminate 911 in which the non-through hole has beenfilled with the conductive paste (see FIG. 9E); superimposing a metalfoil 916 on the laminate 915 from which the mold release film has beenpeeled off (see FIG. 9F); heating and pressing the laminate to allowhollow pores of the porous sheet to be filled with a resin and allow themetal foil to be adhered to the porous sheet, and further compressingand hardening the conductive paste to form an inner via hole 917 (seeFIG. 9G); and forming circuit patterns 920 on the metal foil of a moldedlaminate 919 (see FIG. 9H). The above-mentioned steps are repeated onceor more. By repeating the above-mentioned series of steps and finallyremoving the support base material 907, a circuit board having amultilayer structure and fine patterns can be provided. Moreover, in thestep of FIG. 9A, the resin layer 902 may be formed by the spin coatingmethod, the printing method, or the like. Furthermore, the circuit board901 may use the circuit board of the sixth embodiment (numeral 719 inFIG. 7H). However, a circuit board having another configuration may beused, for example, a circuit board having a through hole, or a circuitboard in which all the layers have an IVH structure. Furthermore, thereis no limitation as to the number of layers.

As mentioned above, in the circuit board using an interlayer connectingtechnique with the inner via hole including a conductive paste, by usingthe circuit board electrically insulating material of the presentinvention, it is possible to provide a circuit board capable of makingthe electrically insulating layer to be homogenized and ultra-thin, andeasily making the connection of an inner via including a paste to bemore reliable. It is possible to provide a circuit board suitable forhigh density mounting. Furthermore, since the circuit board of thepresent invention has an IVH structure and is provided with ahomogeneous and thin electrically insulating layer, the circuit board issuitable for high density mounting. Furthermore, the method formanufacturing the double-sided circuit board and a multilayer circuitboard of the present invention can provide a circuit board that isextremely suitable for high density mounting. Moreover, the wiring layerof each embodiment may contain copper as a main component, the metalfoil to be used for forming the wiring layer may be a copper foil.Furthermore, it is preferable in each method for manufacturing circuitboards that the mold release film is selected from polyethyleneterephthalate (PET), polyethylene-2,6-naphthalate (PEN), and celluloses.Furthermore, it is preferable that the through hole and the non-throughhole are provided by a method selected from a carbon dioxide gas laser,YAG laser, or excimer laser. In particular, the through hole can beprovided by the use of a drill or a puncher. Furthermore, filling of theconductive paste may be carried out by the printing method using asqueegee, etc. However, filling of the conductive paste in a vacuum canincrease the F value of the conductive paste more effectively. It ispreferable that the circuit board is molded by a vacuum heat pressing.The present invention is not necessarily limited to the above-describedembodiments.

EXAMPLE

Hereinafter, the circuit board using the circuit board electricallyinsulating material of the present invention and the circuit boardmanufactured by the manufacturing method of the present invention aredescribed by way of specific Examples.

First, a method for manufacturing a circuit board electricallyinsulating material of the present invention is described.

On one surface of a 25 μm-thick polyethylene terephthalate (PET) filmthat is coated with a silicone based release mold agent, an epoxy resincontaining the below-mentioned compositions ((1) to (4)) was coated bythe use of a bar coater to a desired thickness and dried at 130° C. for15 minutes so as to make the PET film to in a semi-hardened state. Next,a porous sheet was sandwiched between the PET films, and thermallypressed at the softening temperature of the resin (80° C.). Thereafter,only the PET film was peeled off from the laminate and resin layers weretransferred to the both surfaces of the porous sheet. Thus, each circuitboard electrically insulating material was obtained.

(1) brominated bisphenol A epoxy resin (amount of bromine: 23 weight %,epoxy equivalent: 270): 35 parts by weight

(2) trifunctional epoxy resin (amount of bromine: 23 weight %, epoxyequivalent: 270): 35 parts by weight

(3) phenol novolac resin type hardening agent (OH equivalent: 120): 30parts by weight

(4) 2-ethyl-4-methylimidazole: 0.2 parts by weight

Furthermore, the pores of the porous sheet in the below mentionedExamples are continuous pores.

Example 1

A circuit board electrically insulating material was prepared by forminga 13 μm-thick epoxy resin on both surfaces of a porous polyimide film(thickness: 30 μm, porosity: 60 vol. %, average hole diameter: 2 μm). Onboth surfaces of the circuit board electrically insulating material, 12μm-thick PET films were pressed at a temperature of 120° C. and apressure of 2 kg/cm² to form a laminate. Thereafter, a through holehaving a diameter of 75 μm was provided at a desired position of thelaminate by a carbon dioxide gas laser. Next, the through hole wasfilled with a conductive paste containing the following compositions bya printing method, and then the PET films on both surfaces were peeledoff at a temperature of 60° C.

(1) copper powder (average particle diameter: 5 μm): 87.5 parts byweight

(2) bisphenol F epoxy resin: 3.0 parts by weight

(3) epoxy resin in which dimer acid is glycidyl esterified: 7.0 parts byweight

(4) amine adduct hardening agent: 2.5 parts by weight

Then, the laminate from which the PET films had been peeled off wassandwiched by 12 μm-thick copper foils and heated and pressed by athermal pressing method in a vacuum at a pressure of 50 kg/cm² at 200°C. for one hour to form a copper-clad laminate. Circuit patterns wereformed on this copper-clad laminate by the etching method to form adouble-sided circuit board.

Example 2

A double-sided circuit board was prepared by the same method as inExample 1 except that a circuit board electrically insulating materialwas prepared by forming a 13 μm-thick epoxy resin on both surfaces of aporous PTFE film (thickness: 30 μm, porosity: 80 vol. %, average porediameter: 2 μm).

Example 3

A double-sided circuit board was prepared by the same method as inExample 1 except that a circuit board electrically insulating materialwas prepared by forming a 13 μm-thick epoxy resin on both surfaces of aporous aramid film (thickness: 30 μm, porosity: 80 vol. % and averagehole diameter: 2 μm).

Example 4

A double-sided circuit board was prepared by the same method as inExample 1 except that a circuit board electrically insulating materialwas prepared by forming a 40 μm-thick epoxy resin on both surfaces of aporous glass film (thickness: 100 μm, porosity: 80 vol. %, average porediameter: 1 μm) and a through hole having a hole diameter of 150 μm wasprovided on the circuit board by using a drill.

Example 5

A double-sided circuit board was prepared by the same method as inExample 1 except that a circuit board electrically insulating materialwas prepared by forming a 30 μm-thick epoxy resin on both surfaces of anaramid non-woven fabric (“Thermount (R),” product by Du Pont, thickness:80 μm, porosity: 40 vol. % and bulk density: 0.8 g/cm³).

Example 6

A double-sided circuit board was prepared by the same method as inExample 1 except that the 15 μm-thick epoxy resin sheet (NIPPON KAYAKUCO., LTD.) was sandwiched between both surfaces of the porous polyimidefilm (thickness: 30 μm, porosity: 60 vol. %, average hole diameter: 2μm) to form a laminate, and furthermore, 12 μm-thick PET films werepressed onto the both surfaces of the above-mentioned laminate at atemperature of 120° C. and a pressure of 2 kg/cm².

Example 7

A double-sided circuit board was prepared by the same method as inExample 1 except that on one surface of the 12 μm thick PET film, a 15μm thick resin layer (including the above-mentioned compositions) wasformed with a bar coater, and then porous polyimide (thickness: 30 μm,porosity: 60 vol. %, average hole diameter: 2 μm) was sandwiched by theresin layer side of the PET and pressed at a temperature of 120° C. anda pressure of 2 kg/cm².

Comparative Example 1

A double-sided circuit board was prepared by the same method as inExample 1 except that a circuit board electrically insulating materialin which an aramid non-woven fabric (Thermount (R), product by Du Pont,thickness: 80 μm, porosity: 55 vol. %, bulk density: 0.6 g/cm³)impregnated with an epoxy resin was used.

Comparative Example 2

A double-sided circuit board was prepared by the same method as inExample 1 except that a circuit board electrically insulating materialin which an aramid non-woven fabric (Thermount (R), product by Du Pont,thickness: 80 μm, porosity: 40 vol. % and bulk density: 0.8 g/cm³)impregnated with an epoxy resin was used.

Comparative Example 3

A double-sided circuit board was prepared by the same method as inExample 1 except that a circuit board electrically insulating materialin which a porous glass (thickness: 100 μm, porosity: 80 vol. %, averagehole diameter: 1 μm) impregnated with an epoxy resin was used.

Evaluation

Circuit boards of the Examples and Comparative Examples, having circuitpattern in which 500 inner via holes are located in series weremanufactured for evaluation. The resistance value of the via connectionof the circuit board for evaluation and the resistance value of theconnection after the circuit boards for evaluation had been left underthe conditions of 121° C./0.2 MPa/100% RH for 168 hours were measured.The change (PCT) of the values was evaluated. Table 1 shows the results.

First, when the initial connection resistance of the inner via hole wasevaluated, except for Comparative Examples 2 to 3, the resistance valueper inner via hole was an excellent value, 3 mΩ or less, in all cases ofExamples 1 to 7 and Comparative Example 1.

TABLE 1 Initial connecting change rate of connecting resistanceresistance (PCT) Experiment (mΩ/inner %/inner No via hole) via hole)Aspect ratio Ex. 1 4.2  23 0.4 Ex. 2 4.2  24 0.4 Ex. 3 4.0  20 0.4 Ex. 45.5  33 1.0 Ex. 5 5.2  36 1.1 Ex. 6 4.4  21 0.4 Ex. 7 4.1  23 0.4 Co.Ex. 1 5.4  50 1.1 Co. Ex. 2 8.6 150 1.1 Co. Ex. 3 9.8 257 1.0 Ex. =Example Co. Ex. = Comparative Example

The reason the connection resistance values of the inner via hole in theComparative Examples 2 to 3 was high was because the compression rate ofthe electrically insulating material at the time of pressing wasinsufficient. On the other hand, in the case where the same poroussheets as those in Comparative Examples 2 to 3 were used (Examples 4 to5), the resistance of the inner via hole was sufficiently small. This isbecause a paste resin was ejected to a non-woven fabric when theconductive paste was printed, thus increasing the F value and becausethe compressive performance can be secured by the change in thethickness of the sheet due to the impregnation of the resin into theporous sheet at the time of pressing. Therefore, according to thecircuit board electrically insulating material of the present invention,it is possible to provide a circuit board electrically insulatingmaterial that is more excellent in the compressive performance even inthe case where the same reinforcing material is used. Furthermore, inExample 4, even when an inorganic material in which the porous sheetitself is not compressed is used (in Example 4, a porous glass), withthe configuration of the circuit board electrically insulating materialof the present invention, compression can be carried out.

Next, when Comparative Example 1 is compared with Examples 1 to 3 and 6to 7, the initial resistance of the inner via hole is excellent in allthe cases. However, in Comparative Example 1, the change rate (PCT) was50% per inner via hole, while on the other hand, in Examples 1 to 3 and6 to 7, the rate is less than 30%. The connection reliability isextremely excellent. This is because the compressive performance ofExamples 1 to 3 and 5 to 7 is improved with respect to that ofComparative Example 1, and because the conductive paste is ejected tothe porous sheet when the paste resin is printed. Furthermore, inExamples 1 to 3 and 6 to 7, since the thickness of the electricallyinsulating layer is extremely small, and the aspect ratio (ratio ofelectrically insulating layer thickness/diameter of inner via hole) islow, the connection reliability of the inner via hole can be improved.In the case of the porous sheet such as a non-woven fabric, since thediameter of fiber is 10-20 μm, it is extremely difficult to make thethickness of an electrically insulating layer to be about 30 μm.

As mentioned above, according to the present invention, since a highdensity structure having a hole diameter of several μm (porous sheet) isused as a reinforcing material, it is possible to make the circuit boardto be homogenized and thin and at the same time to make the diameter ofthe inner via hole to be small easily. Furthermore, even if areinforcing material in which a porous sheet itself has a smallcompressive performance is used, a circuit board having an inner viahole with a high connection reliability can be provided. Therefore, itis possible to provide a circuit board having an IVH (interstitial innervia hole) structure by using a high density sheet, which has beendifficult in the case of using the conventional material (prepreg usingan aramid non-woven fabric). On the other hand, it is possible to makethe electrically insulating layer to be homogenized and thin by makingthe sheet to be high density.

The invention may be embodied in other forms without departing from thespirit or essential characteristics thereof. The embodiments disclosedin this application are to be considered in all respects as illustrativeand not limiting. The scope of the invention is indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

What is claimed is:
 1. A circuit board electrically insulating materialin sheet form, comprising: a porous sheet; and a resin layer laminatedto a surface of the porous sheet, wherein at least a portion of theresin layer is not impregnated in pores that are present inside theporous sheet, and only a central portion of the porous sheet is notimpregnated with resin from the laminated resin layer and the circuitboard insulating material has a thickness sufficiently small for use ina circuit board.
 2. The circuit board electrically insulating materialaccording to claim 1, wherein hollow pores are present in at least thecentral portion of the porous sheet and the volume of the pores is 10vol. % or more and 45 vol. % or less with respect to the entire volumeof the circuit board electrically insulating material.
 3. The circuitboard electrically insulating material according to claim 1, wherein themaximum hole diameter of the pores is 10 μm or less.
 4. The circuitboard electrically insulating material according to claim 1, wherein theporous sheet comprises one selected from the group consisting of anorganic material and ceramics.
 5. The circuit board electricallyinsulating material according to claim 4, wherein the organic materialcomprises at least one material selected from the group consisting ofpolytetrafluoroethylene (PTFE), polyimide, aromatic polyamide, andaromatic polyester.
 6. The circuit board electrically insulatingmaterial according to claim 1, wherein the porous sheet is a non-wovenfabric containing a synthetic fiber as a main component.
 7. A circuitboard comprising: a plurality of electrically insulating layers andwiring layers which are laminated alternately, and an inner via hole forsecuring an electrical connection between the wiring layers bycompressing and hardening a conductive paste including a conductiveparticle and a resin; wherein: the electrically insulating layercomprises a porous sheet in which a resin layer is laminated to at leastone surface, at least a portion of the resin layer is impregnated inpores that are present inside the porous sheet, and only a centralportion of the porous sheet is not impregnated with a resin from thelaminated resin layer; a through hole, which is provided for making theinner via hole, penetrating the electrically insulating layer in thethickness direction of the electrically insulating layer is filled witha conductive paste including a conductive particle and a resin, andpores that are present inside the porous sheet are filled with resinfrom the laminated resin layer; and the average hole diameter of thepores that are present inside the porous sheet is smaller than theaverage particle size of the conductive particle.
 8. The circuit boardaccording to claim 7, wherein the electrically insulating layer providedwith the inner via hole including a conductive particle and a resin isformed by filling the pores of the porous sheet with a resinsimultaneously with compressing and hardening the conductive paste. 9.The circuit board according to claim 7, wherein the maximum holediameter of the pores of the porous sheet is 10 μm or less.
 10. Thecircuit board according to claim 7, wherein the porous sheet comprisesone selected from the group consisting of an organic material andceramics.
 11. The circuit board according to claim 10, wherein theorganic material comprises at least one material selected from the groupconsisting of polytetrafluorethylene (PTFE), polyimide, aromaticpolyamide, and aromatic polyester.